Currently, rotor brake systems for helicopters and other rotating systems operate in many cases by applying a constant pressure from a discrete valve, which is switched on and off by the operator to one or more hydraulic brake calipers. Under such an arrangement, however, the deceleration profile of the rotor and the specific rotor blade stop-azimuth value are uncontrolled.
Further, helicopters now in production have their rotor heads either manually rotated to a proper angle (via a ground support rope, for example, connected to a tugging point provision on the rotor blade) or hydraulically driven to a proper angle by an indexing, motor/gear assembly.
Other known rotor brake systems, for example, U.S. Pat. No. 4,374,350, issuing on Feb. 15, 1983, to Kolzai et al., include control systems for stopping the spindle of a machine tool at a predetermined rotational position. However, the system shown in this patent employs a proportional magnetic sensor fastened to a rotating shaft in order to locate desired stop angles. The sensor is thus operable over only a small angle, and this renders control nonadaptive to sudden torque disturbances and unschedulable. The arrangement shown in this patent further controls electrical circuitry in the first instance rather than controlling brake mechanics directly.
The arrangement shown in another document, U.S. Pat. No. 4,394,889, which issued on July 26, 1983, to inventor Gray, shows how to stop an elevator (rather than a helicopter rotor blade). This is useful background art, because the elevator stops at a predetermined elevational level. The scheme indicated in the patent uses constant braking pressure, open loop control techniques, and a delay feature based on velocity of the elevator to achieve the selected stop position.
Similarly, U.S. Pat. No. 4,342,378, issued to inventor Hmelovsky on Aug. 3, 1982, is of general interest, as it discloses a technique for stopping an elevator door at a selected position. In this system, discrete positions are selected during door closure to control prescheduled door acceleration and deceleration values. The system accordingly slows the elevator doors to a small velocity and then accomplishes actual stoppage by impact.
Each of the above-indicated, known arrangements either depend upon prescheduled position control laws or permit shaft angle overshoot. For helicopter rotors, such control techniques would only crudely position the rotor, because of the complex acceleration and deceleration effects acting on the rotor blade during braking operation under normal or gusty wind conditions.
For the sake of providing an analytical basis for discussion, it is noted at this point that a helicopter rotor including a selected plurality of blades and having an angular velocity "W" will follow the motion equation: (Wf)(Wf)=(Wi)(Wi)-2 dW(delta P), where "Wf" is the final angular velocity of a selected rotor blade, "Wi" is the initial angular velocity of the rotor blade, "dW" is the rate of change of angular velocity of the rotor, and "delta P" is the angle through which the rotor turns during the evolution from initial to final velocity. According to this relationship, rotor motion is considered stopoed when Wf=0. At that time, (Wi)(Wi)=2 dW(delta P). And further: "delta P" will then equal (Wi)(Wi)/2 dW.